The present invention relates to a process for applying a fluid to a moving web of material and to apparatus for performing this process, having a distributor for the fluid.
The fluid can be a liquid or a gas. In particular, in addition to homogeneous coatings, the process allows uniform wetting or rinsing of rapidly moving webs of material by means of liquids of any kind, such as, for example, water, acid, alkalis or solutions whose ingredients are caused to interact with the surface of the web of material. The web of material is in general a carrier strip, for example, an aluminum strip.
The use of the present process is particularly advantageous in the production and further processing of offset printing plates. For example, the aluminum carrier material for the production of offset printing plates, after degreasing which is carried out with a pickling liquor, is rinsed very uniformly with water in order to avoid pickling spots. Moreover, the carrier material is rinsed in further process steps with surface-active solutions, surface-active ingredients being applied to the surface of the web of material via the wetting of the carrier material. Furthermore, the pretreated carrier material is coated with light-sensitive substances, which are applied in the form of a solvent-containing wet film to the carrier surface, and the solvents are then evaporated, so that the light-sensitive substances alone remain. Uniform wetting is also important in the development of exposed offset printing plates, which are contacted with developer solution in development apparatuses.
Rinsing and/or wetting steps can be carried out in various ways, for example, by means of spray bars which are arranged transversely to the web of material and are equipped with specially designed spray nozzles for distributing the rinsing liquid. The number and shape of the spray nozzles per unit width depends here on the magnitude of the spray volume stream to be applied, the spray liquid being atomized by the nozzle pressure for fine distribution and/or being fanned out across the width of the web of material by a special design of the nozzles. This method is intended to achieve simultaneously continuous wetting of the web of material across the width and a rinsing action.
A disadvantage of spray bars is that, during the atomization, undesirable aerosols are formed, particularly when acid- or alkali-treated webs are rinsed. Furthermore, it is a disadvantage of spray bars that the desired uniform distribution across the width of the web of material can be achieved only within a narrowly limited volume stream range for the rinsing liquid. Uniform rinsing is therefore frequently not ensured in the case of variable speeds of the web of material. In addition, the superposition of the spray cones of the adjacent nozzles leads to undesired fluctuations in the thickness of the liquid film applied, which fluctuations can cause non-uniform chemical reactions.
In coating technology, processes are applied in which slot dies or film coaters produce a liquid film via a short liquid bridge or a free-falling curtain which coats and/or wets the moving web of material without contact. In the case of liquids with low film thickness or with high surface tensions, however, the film curtain frequently tends to have flow instabilities and tears due to constriction and drop formation across the width. The undesirable consequence thereof is unwetted areas on the moving web of material.